c++: Fix tsubsting member variable template-id [PR96330]

This makes tsubst_copy appropriately handle a variable template-id, which
in turn fixes tsubsting a COMPONENT_REF whose member operand is known at
parse time to be a variable template-id, as in the initialization of 'x'
in the first testcase.  Previously, we rejected this testcase with the
error "foo_t::bar<T> is not a function template", issued from
lookup_template_fuction.

We were already properly handling the analagous case where the object
operand of the COMPONENT_REF is dependent (and so the member operand is
a dependent template name), but there doesn't seems to be existing test
coverage for this, hence the second testcase below.

gcc/cp/ChangeLog:

	PR c++/96330
	* pt.c (tsubst_copy) <case TEMPLATE_ID_EXPR>: Rename local
	variable 'fn' to 'tmpl'.  Handle a variable template-id by
	calling lookup_template_variable.

gcc/testsuite/ChangeLog:

	PR c++/96330
	* g++.dg/cpp1y/var-templ68.C: New test.
	* g++.dg/cpp1y/var-templ68a.C: New test.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>

3 files changed, 37 insertions, 3 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 7794137..81df8c8 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -17110,14 +17110,17 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
     case TEMPLATE_ID_EXPR:
       {
 	/* Substituted template arguments */
-	tree fn = TREE_OPERAND (t, 0);
+	tree tmpl = TREE_OPERAND (t, 0);
 	tree targs = TREE_OPERAND (t, 1);
 
-	fn = tsubst_copy (fn, args, complain, in_decl);
+	tmpl = tsubst_copy (tmpl, args, complain, in_decl);
 	if (targs)
 	  targs = tsubst_template_args (targs, args, complain, in_decl);
 
-	return lookup_template_function (fn, targs);
+	if (variable_template_p (tmpl))
+	  return lookup_template_variable (tmpl, targs);
+	else
+	  return lookup_template_function (tmpl, targs);
       }
 
     case TREE_LIST:
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ68.C b/gcc/testsuite/g++.dg/cpp1y/var-templ68.C
new file mode 100644
index 0000000..4c560d4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ68.C
@@ -0,0 +1,15 @@
+// PR c++/96330
+// { dg-do compile { target c++14 } }
+
+struct foo_t {
+  template <class T> static constexpr bool bar = true;
+};
+constexpr foo_t foo{};
+
+template <class T>
+void f() {
+  int x = foo.bar<T>;
+  int y = foo_t::bar<T>;
+}
+
+template void f<int>();
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ68a.C b/gcc/testsuite/g++.dg/cpp1y/var-templ68a.C
new file mode 100644
index 0000000..6091a03
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ68a.C
@@ -0,0 +1,16 @@
+// PR c++/96330
+// { dg-do compile { target c++14 } }
+
+template <class>
+struct foo_t {
+  template <class T> static constexpr bool bar = true;
+};
+template <class T> constexpr foo_t<T> foo{};
+
+template <class T>
+void f() {
+  int x = foo<T>.template bar<T>;
+  int y = foo_t<T>::template bar<T>;
+}
+
+template void f<int>();